The mechanisms of upper airway obstruction during NREM sleep are not well-understood. Increasing evidence suggests a relationship between breathing instability and upper airway obstruction in susceptible individuals. This protocol will examine the effect of several chemoreceptor perturbations, such as hypoxia, hypercapnia, and hypocapnia on the breathing instability and upper airway obstruction during NREM sleep. The study population will cover a wide spectrum of susceptibility to collapse from normals to snorers and patients with obstructive sleep apnea.